Gases have been injected into oil reservoirs to improve oil recovery, and have been utilized in both immiscible and miscible flooding techniques. Under immiscible conditions, the injected gas delays pressure declines and displaces the oil toward producing wells, while the miscible technique has the advantage of essentially eliminating the interfaces and capillary forces between the oil and the miscible agent. Various hydrocarbon gases and liquids have been used as miscible displacement agents; however, such reagents are no longer economical.
Carbon dioxide is a favorable alternative to hydrocarbon reagents, since upon initial injection the carbon dioxide is not miscible with the oil; however, because of its ability to vaporize hydrocarbon components in the oil, a miscible mixture between the reservoir oil and carbon dioxide is formed. The dissolved carbon dioxide volumetrically expands the oil and reduces its viscosity which enables the oil to flow more readily and results in better recovery.
The use of carbon dioxide has great potential for recovering oil using the miscible flooding technique; however, the realization of such potential will depend in large measure upon the availability of low cost carbon dioxide. Usual sources of carbon dioxide, such as naturally occurring deposits, flue gases from power plants and the like contain hydrogen sulfide which must be removed to be in compliance with local ordinances. Thus, for example, the acceptable hydrogen sulfide content in a carbon dioxide slug for enhanced oil recovery is about 100 ppm by volume, which is far below the usual hydrogen sulfide content in a carbon dioxide gas stream available, for example, from natural gas deposits.
The efficiency of conventional processes for hydrogen sulfide removal is significantly affected by the amount of carbon dioxide present. For example, since the conventional Claus reaction for hydrogen sulfide removal involves the reaction of hydrogen sulfide with sulfur dioxide to yield elemental sulfur and water proceeds in a reversible fashion, the carbon dioxide which functions as a diluent adversely shifts the Claus equilibrium. Accordingly, the Claus process is generally used for acid gas streams containing 20 volume percent hydrogen sulfide or greater.
Attempts to remove the hydrogen sulfide from the carbon dioxide gas stream have resulted in the undesirable production of reaction products of the carbon dioxide and hydrogen sulfide as well as between any hydrocarbon gases present to form additional impurities which are unacceptable.